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Where to hook up for fuel pressure

C

Charles in SC

Well Known Member
I have an IO-360 M1B. For fuel pressure gage hook up do I hook to the elbow with the hole in it at the fuel pump? The plans show this. The reason I ask is because there is a fitting on the fuel spider with a plastic cap on it.
Thanks in advance!
 
Charles in SC said:
I have an IO-360 M1B. For fuel pressure gage hook up do I hook to the elbow with the hole in it at the fuel pump? The plans show this. The reason I ask is because there is a fitting on the fuel spider with a plastic cap on it.
Thanks in advance!
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Right, the pressure tap is at the fuel pump exit fitting. There is an extra port on the spider for a fuel pressure indicator, but it is really for fuel flow. It's pressure is an indicator of fuel flow rate. Not as accurate as the red cube. I just put a plug in it.

You want fuel pressure to the servo to be measured.
 
Just make sure you take the pressure off the DISCHARGE side of the pump, and not the supply side. Yep---we've seen that too.
Tom
 
Don't use the diaphragm vent hole! You must pick up your fuel pressure reading on the outlet side of the pump. Generally it will be a fuel fitting with an "auxiliary" outlet to connect your fuel pressure hose/sending unit.
 
Here is a typical set up. The plated fitting is the O ring type, can't recall the part number. The darker fitting is a standard steel fitting. One goes to the servo, the other to pressure sensor. There are lots of combinations and orientations depending on your set up.
sxch0m.jpg
 
Barneybc12d said:
What is the function of the small fitting/tube in the picture?
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It is a vent line. Also, If the diaphram gets a pin hole, the vent allows the pump to still work instead of the space between the diaphragms filling with fuel.
That is a typical fuel pump failure and a blue stain out of the vent line will give you a warning before you end up in a ditch.
 
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Fuel pressure

so if my fuel pressure is low is there a way to adjust it. I'm getting good fuel flow but during a long flight it can drop to as low as 13 psi eventually I'd comes back up to 21. Any thoughts ? It's a Lycoming IO-360
 
AttackPilot64 said:
so if my fuel pressure is low is there a way to adjust it. I'm getting good fuel flow but during a long flight it can drop to as low as 13 psi eventually I'd comes back up to 21. Any thoughts ? It's a Lycoming IO-360
Click to expand...

Pressure is fixed by an internal regulator (or spring force). 100LL or mogas?

It maybe as the tanks get lower, heat soak occurs, and with altitude , the fuel light ends begin to outgas slightly and diminish the pumping efficiency. But purely speculation. Hot days, mogas (low vapor pressure), and altitude would be more prone to this.

Fuel vapor pressure, temperature at the pump inlet, and suction head at the pump inlet are the key dimensions to evaluate the condition.
 
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